The present invention relates to a color cathode ray tube.
In general, a color cathode ray tube comprises an envelope. The envelope has a substantially rectangular panel provided with a skirt portion at the peripheral portion of an effective surface of a curved surface, and a funnel joined with the skirt portion. On the inner surface of the panel is formed a phosphor screen which includes light absorbing layers and three-color phosphor layers formed at gaps between the light absorbing layers. An electron gun is arranged in the neck of the funnel and a deflector is mounted on the funnel. Further, a substantially rectangular shadow mask is provided in the envelope to be opposed to the phosphor screen.
In this color cathode ray tube, three electron beams emitted from the electron gun are deflected by the deflector and horizontally and vertically scan the phosphor screen through electron beam apertures of the shadow mask, thereby to display a color image.
In general, a shadow mask is comprised of a substantially rectangular mask body and a substantially rectangular mask frame. The mask body has a main surface portion consisting of a curved surface opposed to the phosphor screen and having a number of electron beam apertures formed therein, and a skirt portion provided at the peripheral edge of the main surface portion. The mask frame has side walls contacting with and welded to the skirt portion of the mask body.
With one of methods of supporting the shadow mask, substantially wedge-like elastic support members are fixed to the side walls at the corners of the mask frame, and the support members are engaged with stud pins provided at the skirt portion of the corners of the panel, thereby detachably supporting the shadow mask on the inside of the panel.
In the color cathode ray tube as described above, three electron beams which have passed the electron beam apertures of the mask body must properly land on the three color phosphor layers, respectively, in order to display an image without color deviation. Therefore, it is necessary to arrange the shadow mask at a proper alignment relationship with respect to the panel.
Recently, in a color cathode ray tube such as a display tube used in a terminal apparatus of a computer in response to introduction of multi-media, the arrangement pitch of three color phosphor layers is reduced in comparison with a normal color cathode ray tube, to improve the resolution, and therefore, the margin rate for beam landing is so small that color deviation easily occurs. Accordingly, more precise beam landing is required.
However, actually, in steps of manufacturing a color cathode ray tube, the shadow mask is repeatedly attached to and detached from the panel, and during the attaching and detaching operations, a stress applied on the shadow mask causes the mask frame to be distorted. The distortion of the mask frame further causes the mask body to be distorted, and as a result, beam landing is shifted from the predetermined three color phosphor layers, thereby causing color deviation.
In addition, in the cathode ray tube as described above, since the margin rate for beam landing is small, it is necessary to reduce the position shifting of the shadow mask caused by any external impact as much as possible. To reduce the position shifting of the shadow mask caused by an external impact, it is also necessary to reduce loads acting on the elastic support member itself. However, if the weight of the mask frame itself is decreased to reduce loads on the elastic support member, the mechanical strength of the shadow mask against a stress applied thereto when attaching and detaching the shadow mask in steps of manufacturing a color cathode ray tube is lowered. As a result, the position shifting of the shadow mask is caused with respect to the panel.
Further, with respect to the shadow mask, in order to prevent unevenness of a formed phosphor screen, the mask body is thinned so as to reduce variations of the shapes and sizes of the electron beam apertures of the mask body, thereby reducing unevenness of the phosphor screen due to variations of the electron beam apertures. However, if the mask body is thinned, the mechanical strength thereof decreases and tends to be deformed easily.
Also, the shadow mask is heated by collisions of electron beams and expands, causing doming. In a shadow mask formed of a low expansion-coefficient material such as invar which efficiently restricts registration of beam landing caused by the doming, a mask frame provided with a convex portion (or bead portion) has been proposed as a counter measure against deformation.
However, not only the mask frame of a conventional shadow mask but also the mask frame having the convex portion as described above is short of suitable mechanical strength and cannot respond sufficiently to variations of mask frames in manufacturing steps and deformation in assembling shadow masks.